1. Field of the Invention
The present invention relates generally to the field of measuring and monitoring the heating value of gaseous fuels such as natural gas and, more particularly, to a method and apparatus for determining and monitoring the Wobbe Index of such fuels on a real-time basis.
2. Description of the Prior Art
The heating values of gaseous fuels such as those largely containing alkane hydrocarbons are frequently given assumed average numbers; thus the heating value of natural gas, for example, is frequently assumed to be 1000 British Thermal Units (BTU's) per cubic foot. In the past, pricing of such fuels has been based upon either the assumption of a nominal average value or by the periodic checking of the actual value by a variety of time-consuming methods.
In one method chromatographic analysis of the constituents has been used to compute the actual heating value or BTU content of a given natural gas from the percentage composition of the mixture. In another method the heat content has been determined by measuring the amount of heat liberated in burning exactly one cubic foot of gas (saturated with water) at standard conditions of temperature and pressure. The heat so liberated is absorbed by a weighted amount of water and the subsequent temperature rise of the water used to calculate the gross or "higher heating value".
These methods involve reasonably expensive instrumentation and require considerable labor to perform the measurements and calculations. Such testing, of necessity, introduces considerable time delay and certainly appears less desirable than an on-line, real-time system. Even so, these might be sufficient methods if, in fact, the composition of the gas being used did not vary greatly with time. However natural gas, for example, may vary greatly in composition depending on the gas fields from which it came and the treatment it receives before distribution. The gas that reaches the customer consumers frequently is only about 85% methane with the remaining 15% being a mixture of various hydrocarbon molecules such as ethane, propane, n-butane, i-butane, etc. Also, as much as 25% of the gas reaching the customer may be made up of non-combustible constituents which occur naturally or have been added to the mixture. These include nitrogen, air and carbon dioxide. Natural gas is used herein as a representative example because it is by far the most widely used gaseous fuel.
The inerts, of course, add nothing to the heating value, and the heating value of alkane, and other hydrocarbons of higher order than methane have a higher heating value on a volumetric basis because of their higher molecular weight. In view of the great variation in constituents of natural gas, the heating value even in a single distribution system may vary greatly with time.
As a result of these and other factors, a random sampling of the heat content of the natural gas being distributed, might lead to great inaccuracies as to the actual heating value of the fuel delivered. As the price of natural gas has increased greatly, it has become necessary for the heat content of the natural gas to be continually monitored and adjusted in order to stay within promised specified limits and to insure that the user is charged for the proper amount of heating value he receives from the fuel.
One such on-line device for the continual monitoring of the higher heating value of such fuels is illustrated and described in present pending application Ser. No. 105,794 filed Dec. 20, 1979, and now abandoned, by William B. Kude and A. Noel J. Pearman, co-inventors in the present application, and Daniel L. Youngbauer. That application is assigned to the same assignee as the present invention.
By that invention there is provided an on-line fuel BTU meter which includes a precise, adjustable metering system which accurately proportions an amount of fuel gas or calibration gas to be tested with a known amount of air such that, at any given time, the volumetric ratio of air to fuel is precisely known. The mixture is fed to a burner system in which the fuel is combusted in the presence of a solid-state ceramic electrochemical cell which provides a step-change in voltage output as the amount of residual oxygen or combustibles, a certain repeatable amount, which, in the preferred embodiment approaches zero, that is, approaches the point of stoichiometry. An electrical signal from the electrochemical cell is utilized with a programmable electronic data processing system to adjust the fuel mixture in accordance with the signal output of the electrochemical cell to achieve and maintain the stoichiometric air-fuel ratio as signalled by the rapid change in electrical output of the cell at that point. The air-fuel ratio at that point is known from the measuring system and the heat content is known from the measuring system and the heat content of the fuel is readily determined based on the known constant relationship between the stoichiometric air-fuel ratio and the fuel heat content.
Whereas the BTU meter of the above-mentioned patent application presents a highly accurate on-line system for monitoring the heat content of gaseous hydrocarbon fuels on a volumetric basis, another very important dimension should be added to account for changes in the specific gravity of the fuel so that a constant heat input to a pressure fueled burner system can be maintained. Thus, the utility companies are very much concerned that the customer be charged precisely for the amount of heat value sold on a volumetric basis, those utilizing the gaseous fuel are also concerned that the specific gravity be considered so the burner energy input remains constant, whatever the relative BTU content be on a volumetric basis. Variations in burner efficiency can be very costly especially in industrial burner applications.
Accordingly, changes in the density or specific gravity of the fuel should be considered in addition to the heating value of the fuel on a volumetric flow basis in order to assure constant heat input to the burners. Such may be accomplished by determining and monitoring the Wobbe Index of the fuel which is defined as follows: EQU Wo=Hv(sg).sup.-0.5
where:
Hv is the higher heating value of the fuel in BTU's/ft.sup.3 and
sg=the specific gravity of the fuel.
It is apparent from the above that by considering changes in the specific gravity of the fuel gas in addition to the heating value on a standard volumetric basis, the Wobbe Index is useful as a measure of the potential heat production available for a given burner input. Thus, a fuel mixture of constant Wobbe Index will provide constant heat input to a burner system. Because of the great variation in the heating value of gaseous fuels the Wobbe Index has long been used in connection with providing a more constant heating value input to burner systems to produce better system stability. In the past, however, in order to determine the Wobbe Index of a fuel gas, separate calorimetry and density measurements had to be made and the results combined. This involved the use of expensive calorimeters and density meters and required considerable time to obtain usable results. It was not possible to obtain the desired real-time Wobbe Index Control in an industrial setting.
In the prior art, attempts have been made to measure perameters related to the Wobbe Index and use these to, in turn, monitor or control burner input. One such prior art scheme is found in a patent to Krijgsman, European Patent No. 0 008 151, issued Feb. 20, 1980. That invention basically uses the oxygen measurement of the products of combustion of a completely combusted sample at constant volumetric air and gas flow to correlate to the Wobbe Index. A fuel sample is withdrawn from a sample stream containing a flow nozzle in a manner in which the pressure drop across the flow nozzle and thus the flow is kept constant. The sample is burned in a combustion chamber combined with a control stream of combustion air. The oxygen content of the products of combustion is measured and the Wobbe Index is derived from the measured oxygen content. Thus, the device operates at known constant sample flow and known constant oxygen supply to the burner and the Wobbe Index is mathematically correlated with the oxygen measurement in the exhaust gas. The preferred mode of operation is with an excess amount of oxygen in the exhaust gas, for example, at 5% oxygen. The Wobbe Index is thus derived directly from the oxygen content of the products of combustion. The measurement apparatus of that invention is utilized to control the mixing apparatus by means of which gases having different Wobbe Indices can be combined to produce a heating gas with a constant Wobbe Index.
Another prior art device is illustrated and described in a patent to De Livois, U.S. Pat. No. 3,783,684 issued Jan. 8, 1974. That invention is concerned with providing constant fuel heat content flow in a burner system. The device utilizes a calorimeter system in which the fuel input is adjusted such that the temperature rise in the calorimeter remains constant for a constant amount of supplied combustion air. In this manner the fuel supplied to an entire combustion system may also be adjusted based on the fuel necessary to keep the calorimeter temperature rise at a given desired value. This is accomplished by pressure drop control across supply orifices, thus, that invention presents a direct control system based on calorimetry rather than a system for measuring the actual Wobbe Index of the fuel.
While these and other prior art devices have been somewhat successful in providing on-line-measurement of Wobbe Index related perameters, they lack a single accurate reference point from which to base such measurements.